Generally, fullerenes are spherical molecules composed of 5- and 6-membered rings, which are attached to each other and have carbon-carbon double bonds. Due to their unique structures, fullerenes are highly applicable in industrial fields, including medical fields related to diseases such as Alzheimer's disease, or free radicals, and nanotechnology fields.
In particular, buckminster fullerene (C60) is a stable carbon molecule consisting of 60 carbon atoms and having a spherical structure and possesses an icosahedral symmetry, and thus all the carbon environments are identical. This was confirmed by a single peak in C-NMR. In addition to the C60 fullerene, C70 and C80 fullerenes are also synthesized, but the amount of synthesis thereof are not large, and thus the C60 fullerene has been mainly synthesized and studied.
Fullerene molecules are symmetrical non-polar molecules having a substantially spherical structure and do not dissolve in polar solvents such as water or alcohols and easily dissolve in non-polar solvents such as benzene or toluene. Further, fullerenes are very sensitive to light and can be easily converted to radicals or light sensitizers, because they become an excited state when they exposed to light. The electrochemical properties of fullerenes are very useful, and the fullerene molecules can undergo 6 reversible oxidation-reduction reactions and are very hard because of their unique structures. In recent years, it was reported that fullerene molecules can be converted to superconductors when they are mixed with alkali metals, and thus the industrial applicability thereof is high.
As described above, fullerenes are very sensitive to light, and the light absorbance in the UV region (213, 257 and 329 nm) is relatively high. However, the fluorescence of fullerenes is known to be very low and can be given by the fluorescence quantum yield. The term “fluorescence quantum yield” refers to the number of emitted photons relative to the number of absorbed photons, and the quantum yield of fullerenes is shown to be about 1×10−4 at room temperature. Due to this low fluorescence efficiency, the fluorescence of fullerenes has not been applied.
In addition, fullerenes have unique optical and electrical properties, but there are several difficulties in applying fullerenes. The difficulties include low solubility in organic solvents, self-aggregation phenomena, etc. In particular, fullerenes are very sensitive to surrounding environments, and thus the physical and chemical properties thereof easily change. To control such characteristics, various methods have been developed.
To apply fullerenes in the biological field, the medical field, the nanotechnology field and the like, it is required that fullerenes easily dissolve in water (water-soluble properties). Thus, it is particularly important to prepare water-soluble fullerenes.
Typical methods for preparing water-soluble fullerenes include a method of preparing fullerol by introducing a hydroxyl group (OH) to the surface of fullerene through a chemical reaction [Tetrahedron, (1996) 52, 4963-4972; Chem. Comm., (1993) 1784; J. Mater. Chem., (2005) 15, 1049], a method of introducing oligoethylene glycol or polyethylene, modified with a ligand capable of binding to fullerenes [Langmuir, (2006), 22, 5366-537; Polymer, (2007) 48, 1972-1980; Bioconjugate Chem., (2008) 19, 2280-2284], etc. However, such methods have problems in that these preparation methods are complicated, because many synthesis steps are required for binding to fullerenes, and the utility of the prepared fullerenes is significantly low, because the solubility thereof in water is not high.
Thus, many researchers are recognizing that developing water-soluble fluorescent fullerenes having high solubility in water and unique optical properties (such strong fluorescence) by a simple reaction is important in increasing the utility of fullerenes in the biological field, the medical field and the nanotechnology field.
Accordingly, the present inventors have made extensive efforts to solve the above-described problems occurring in the prior art and, as a result, have found that a water-soluble fullerene derivative having strong fluorescence can be prepared in a simple and easy manner by mixing fullerene with a ligand having a terminal hydroxyl group in a first solvent and then reacting the mixture in the presence of a catalyst, thereby completing the present invention.